The Sunday 3am incident was carried out by a group of about 10 heavily armed militants.
The reservist was hit by bullets during the battle to repulse the attackers.
Mr Marete said the reservists managed to repulse the attackers after an intense gun battle.
During the incident, a boy aged about 10 suffered a gunshot wound in the hip.
Suspected Al-Shabaab militants have killed a police reservist in Mandera County in an attack seen have been targeting to destroy a communications mast.
The Sunday 3am incident was carried out by a group of about 10 heavily armed militants, according to Lafey Deputy County Commissioner John Marete.
